I've had the airlock clog on very active ferments, and the pressure from the gas will blow the top off. I've had it happen at least twice that I remember. It's a pretty violent explosion, but I didn't have to clean up much beer. It does spew yeast all over the place, which is messy to clean. One time, in my apartment in Baltimore, I was awoken by the bang. When I went into the room, which had ~14' ceiling (and these awesome windows and bookcases), the airlock had exploded upward with such force that it left a giant dent in the ceiling. The lid was laying about 6 feet away, and there was yeast coated over my books.
Take-away? You can actually not put the lid on your bucket, but cover the ferment with a towel while it is active. Nothing will drop in (because of the towel), and that lets out the gases very efficiently with no chance of getting clogged. I always do this with a seriously top-cropping yeast. The yeast for the Baltimore explosion was WLP570, and I found that fermenting that yeast in particular warm, and 'open' (with a towel) actually made for a flavor profile that I liked better. But at 69-70F, that yeast would produce enough of a krausen that it overflows a 7 gallon fermenter on a 5 gallon batch.